Various types of ultrasonic transducers have been developed for transmitting and receiving ultrasound waves. These ultrasonic transducers are commonly used in many applications including medical diagnostics and therapy, sonar and underwater imaging, biochemical imaging, non-destructive evaluation of materials, communication, proximity sensing, gas flow measuring, in-situ process monitoring, acoustic microscopy, and a variety of other uses. Ultrasonic transducers may be produced and/or used as single, discrete transducers. Further, ultrasonic transducer arrays that contain multiple transducers have also been developed. For example, two-dimensional arrays of ultrasound transducers can be used for 3-D imaging and other applications.
One type of ultrasonic transducer that has been developed is the micromachined ultrasonic transducer (MUT). Compared to the widely used piezoelectric (PZT) ultrasound transducer, the MUT has advantages in fabrication methods, operation bandwidth and operating temperatures. For example, PZT transducers are typically produced by making arrays of transducers, dicing the arrays, and connecting individual piezoelectric elements. This manufacturing technique can be fraught with difficulty and expense, and the PZT transducers themselves may have a large impedance mismatch problem. On the other hand, MUTs can be manufactured using more efficient semiconductor micromachining techniques, and MUTs demonstrate a comparable dynamic performance to PZT transducers. For these reasons, the MUT is becoming an attractive alternative to the piezoelectric ultrasound transducer.
One type of MUT that is widely used is the CMUT (capacitive micromachined ultrasonic transducer), which uses electrostatic attraction between electrodes. For example, a CMUT having embedded springs and surface plates (ESCMUT) has been recently developed to improve device performance. The basic structure of an ESCMUT is a surface plate or multiple surface plates that are supported by embedded springs in a micromachined structure. However, while the CMUTs developed thus far demonstrate better performance than PZT transducers in most device parameters, CMUTs may still be improved to enable CMUTs to outperform PZT transducers in every application field.